Many of the migrants appeared to have been trapped in the overcrowded vessel's hold when it capsized, a security official in the Libyan town of Zuwara, from where the boat had set off, said.

The Libyan coast guard rescued around 200 people, who were later taken to a detention facility in Sabratha, west of Tripoli, the official said.

The migrants on board were from sub-Saharan Africa, Pakistan, Syria, Morocco and Bangladesh.

Earlier, the Swedish navy ship Poseidon found the bodies of 51 migrants found suffocated below deck of a boat off Libya's coast.

Poseidon was already rescuing 130 migrants from a raft when it got a call to assist a nearby wooden vessel, authorities said.

Zuwara, Libya's most western town located near the Tunisian border, is a major launchpad for smugglers shipping migrants fleeing conflict and poverty to Italy.

Cross-border smuggler networks exploit the country's lawlessness and chaos to bring Syrians into Libya via Egypt or nationals of sub-Saharan countries via Niger, Sudan and Chad.

More than 2,300 people have died so far this year in attempts to reach Europe by sea, compared with 3,279 in the past 12 months, according to the International Organisation for Migration.

It comes as thousands of migrants and refugees a day are travelling between Greece to Hungary as they seek a better life in the European Union.

They are taking the perilous "Balkan Corridor", with many of the countries on the way feeling the strain of so many people passing through.

Hungary has told Sky News that "swift action" must be taken in order to tackle the crisis.

While many have said they are heading to Germany, and some to the UK, a large number claim they are on their way to Sweden where the country's tolerant approach to immigration is being put under strain.